I agree with you that Chelsea's last win cannot be used as a benchmark because they are in a better position I mean playing against 9 players if that is a normal match maybe the results will be different, as you said if Chelsea are able to go through the next 2-4 games without defeat, it can be said Chelsea in a positive trend. I personally doubt Chelsea can get 4 points in the next 4 matches unless they get the same luck as against Tottenham, but we know that luck does not come at all times and can't always hope for that situation.
This season the fight for European League tickets seems more interesting than the Champions League ticket, we already know which teams will secure UCL tickets but for the European League Premier League teams have many competitors including Manchester United, Chelsea and Newcastle there is also Aston Villa who is still consistent which can sometimes surprise the opponent.
Tottenham where so unfortunate in the game and the red card booking gave Chelsea more advantage in the match even if Tottenham were playing against a team like Luton Town or Fulham I bet they would've lost the match cause it would be so difficult to win a match with 9 men against 11, Chelsea were so lucky in the match and their striker Jackson really utilized the opportunity to get a hat-trick and help Chelsea secure the 3points.
However I doubt if same luck would shine one them against City, Chelsea might be playing with a home advantage but I doubt if they'll be able to hold of Pep Guardiola's side from getting the 3points, maybe their confidence level might have been boosted based on their performance against Tottenham but is it going to be enough to hold off City who have a better squad.